a) Field of the Invention
This invention relates to a process for the dimerization of n-butene, and more specifically to a process for dimerizing n-butene at a high conversion so that isooctene having a low degree of branching can be obtained with high selectivity.
Isooctene having a low degree of branching is industrially useful as a raw material for alkylphenols, intermediates for surfactants. It can also be hydroformylated into an alcohol and then esterified with carboxylic acids. These esters are also industrially useful as raw materials for plasticizers which are suitable for use in vinyl chloride resins and the like.
When isooctene having such a low degree of branching is used, the final products have good biodegradability and can give compounds particularly preferable from an environmental viewpoint.
b) Description of the Related Art
Isooctene is obtained by dimerization of butene. Isooctene obtained by dimerization is generally a mixture of isooctene isomers having different degrees of branching and/or containing the ethylenically unsaturated bond at different positions. They are used as a mixture without distillation, i.e., isolation. In particular, isooctene having a low degree of branching is indispensable as a raw material for final products of high biodegradability. Only isooctene having a high degree of branching, however, is available from conventional technology resulting in various inconveniences. For example, it has been pointed out that a low polymer having a high degree of branching has poor biodegradability and, when employed as a plasticizer in food wrap films made of vinyl chloride resin, is taken into the body.
As dimerization processes of olefins, processes making use of an anionic polymerization catalyst, a cationic polymerization catalyst or a coordination polymerizing catalyst have been known for many years. Many of these known processes, however, relate to low polymerization of ethylene or propylene. Even if it is attempted to apply these processes for the dimerization of butene, catalytic activity is not observed at all or, even if some catalytic activity should be observed, the degree of branching of the resulting isooctene is too high to provide the desired quality.
Ni-containing Ziegler catalysts are used with preference to prepare isooctene having a low degree of branching because they are active or are superior in the quality of the butene dimer. Processes in which nickel is not used as a primary catalyst are also known. Different from the present invention, the reactions of these processes are all heterogeneous reactions. As processes for preparing a butene dimer having a low degree of branching, there have been disclosed, for example, the process in which butene is dimerized using ethylaluminum dichloride and ethyl-2-hexanoic acid trifluoride (Japanese Patent Application Laid-Open No. 36493/1981) and the process in which butene is dimerized using alkylaluminum chloride and a fatty acid nickel compound as catalysts (Japanese Patent Application Laid-Open No. 157510/1979).
The catalyst systems, however, have extremely low activity in these processes. Reference may be made, for example, to Example 6 of Japanese Patent Application Laid-Open No. 36493/1981, where in the dimerization of mixed butene composed principally of 2-butene and n-butane, the conversion of the butene was 66% in 2.5 hours and 75% in 5 hours. Reference is also made to Example 1 of Japanese Patent Application Laid-Open No. 157510/1979, where in the dimerization of mixed butene composed of n-butene, isobutene, n-butane, etc., the conversion of a butene was 50-80% in the reaction which lasted 5-7 hours. A long reaction time is therefore needed. Moreover, the selectivity to the dimer was not higher than 85%.
Known processes also include those using a polyol, alcohol or aliphatic diol as a third additive. These processes, however, neither disclose nor suggest the use of a nitrogenate as an additive in a homogeneous nickel catalyst system, which is a characteristic feature of the process according to the present invention. Moreover, no substantial activity improvement is observed in the conventional processes which require the addition of a third additive.
When a catalyst other than nickel is used, it is known to use a basic nitrogenate as a third additive. For example, Japanese Patent Application Laid-Open No. 29503/1975 discloses a process in which propylene is dimerized using a catalyst composed of potassium, copper and an aliphatic amine. Further, for example, Japanese Patent Application Laid-Open Nos. 11801/1974, 11802/1974 and 11803/1974 disclose processes in which an olefin is dimerized using an alkylaluminum halide, a nitrogenate and a tungsten compound. Even if these processes are applied to the dimerization of butene, no catalytic activity is exhibited.
On the other hand, Japanese Patent Application Laid-Open No. 118423/1980 discloses a process for the dimerization of an 1-alkene, in which titanium, an alkylaluminum halide and a nitrogen-containing Lewis base are used. It is disclosed in this Japanese Patent Application Laid-Open No. 118423/1980 that the presence of the nitrogen-containing Lewis base can minimize polymerized products of olefins such as propylene and permits selective preparation of a dimer. It is, however, to be noted that the process disclosed in Japanese Patent Application Laid-Open No. 118423/1980 suppresses with the basic additive the catalytic activities of titanium and the alkylaluminum halide, both known to have sufficient polymerization activity, and terminates the reaction at the dimer stage. Namely, the selectivity to the dimer is improved but the conversion of the olefin drops significantly. The present inventors applied this process to butene. As a result, this process has been found not to be a fully advantageous process because it requires an expensive titanium catalyst in a relatively large amount, the catalytic activity is lower compared to that in the process known from Japanese Patent Application Laid-Open No. 157510/1979 and, if titanium and the alkylaluminum halide are used in greater amounts to improve the catalytic activity, the selectivity to the dimer drops.